Process for preparing semiconductor ingots within a depression



July 19, 1966 w. KELLER ETAL PROCESS FOR PREPARING SEMICONDUCTOR INGOTS WITHIN A DEPRESSION 2 Sheets-Sheet 1 Filed NOV. 29, 1963 FIG. 1

FIG. 2

PROCESS FOR PREPARING SEMICONDUCTOR INGOTS WITHIN A DEPRESSION 2 Sheets-Sheet 3 Filed Nov. 29. 1963 FIG. 3

United States Patent 3 Claims. (31. 148-16) Our invention relates to a process for preparing semiconductor rods, and particularly to growing monocrystalline semiconductor ingots by drawing out of a melt.

Such a method comprises melting a semiconduoter in a crucible under a non-reactive atmosphere, dipping a single-crystal seed into the melt surface, and slowly withdrawing the seed. Melt material solidifies on the seed in a crystal orientation determined by the seed and in the shape of a rod larger than the seed. This method was taught by Czochralski.

Other methods of producing monocryst alline semiconductor rods are known. For example, crucible-free zone melting for growing monocrystalline rods was taught by Theuerer. The pedestal method is disclosed in Growth and Perfection of Crystals, by Doremus, Roberts and Turnbull, published by John Wiley & Sons, Inc., New York, and Chapman and Hall, Ltd., London 1958. According to the latter method, induction heating produces a molten drop on a slit semiconductor rod. A monocrystalline seed is inserted into the drop and a monocrystalline rod is drawn out.

In drawing a rod out of a quartz or graphite crucible the diameter of the grown semiconductor material can be controlled by regulating the melt temperature, regulating the drawing speed, or both. Thus, a small seed monocrystal can be grown to a monocrystal of larger diameter. However, in this process impurities such as oxygen may diffuse into the melt from the crucible wall. Also when drawing rods of materials having high melting temperatures, the melt temperature may plastically deform the crucible wall. On the other hand, crucible-free zone melting only permits growing single-crystal rods of small diameter; diameters greater than mm. being obtainable thereby only with considerable difficulty. Producing single crystals of diameters greater than mm. with the zone-melting process is essentially impossible.

It is an object of this invention to provide an improved method for producing monocrystalline semiconductor rods.

It is a more particular object of our invention, regarding growth of monocrystalline semiconductor rods, to provide an improved method which avoids the abovementioned difficulties inherent in the methods heretofore known.

Another object of the invention is to provide a simple method for growing semiconductor rods of large diameters.

According to a feature of our invention, we form a depression in a semiconductor block and produce in the depression a melt of the same material as the block. We then draw a semiconductor rod upwardly out of the depression in the block.

According to another feature of our invention, we supplement the amount of material in the melt by continuously advancing into the melt new semiconductor material having a mass equal to the mass drawn out of the melt so that the level of the melt remains constant.

These and other features of novelty characterizing the invention are pointed out in the claims. Other objects and advantages will become obvious from the following detailed description when read in light of the accompanying drawing wherein:

FIG. 1 is a perspective view of an apparatus performing the method of the invention; and

FIG. 2 is a cross-sectional view of FIG. 1.

FIG. 3 is a partially schematic, partially sectional, and partially perspective view of another apparatus performing the method of the invention.

In FIG. 1 a semiconductor block 2 defines :a depression or recess R whose bottom supports a melt 3 of the same semiconductor material. Two high-frequency coils 4 and 5, energized by a generator G, inductively heat the melt. Known means, shown as 10, draw a monocrystalline semiconductor rod 6 upwardly from out of the melt through heating coil 5. Rod 6 is started with a monocrystalline seed which first is dipped into the melt so that the entire rod as it is drawn becomes monocrystallinc.

A polycrystalline semiconductor rod 7 of the same material as rod 6 is directed downwardly by other known means 11, through the heating coil 4 which melts the bottom of the rod, and into the melt. Both semiconductor rods 6 and '7 have the same diameter in FIGS. 1 and 2. The rod 7 is advanced downwardly with the same speed that the rod 6 is withdrawn, so that the melt mass remains constant. According to the invention, the entering rod 7 may have a smaller or larger diameter than rod 6. In that case, the melt mas-s is maintained constant by regulating the advancing speed of rod 7.

According to another embodiment of the invention, the semiconductor material 7 is added to the melt in the form of a powder or granular semiconductor material. Of course it is necessary to introduce the semiconductor rnaterial only after being converted to fiuid form as illustrated for example in FIGS. 1 and 2 where the downwardly advancing rod 7 is melted by induction coil 4 so that the semiconductor material from rod 7 enters the melt in fluid form.

The semiconductor rod 6 in FIGS. 1 and 2, as an example of the embodiment of the invention, has a diameter of approximately 40 mm. The melt in the case of silicon has a semiconductor mass of about grams. The withdrawing speed of the rising rod 6 is 2 to 4 mm. per minute, preferably 3 mm. per minute. The entire operation must be carried on in an inert atmosphere which is provided in the usual manner.

The semiconductor block 2 preferably is prepared by grinding semiconductor rods or particles into prism shapes matching each other and, after etching away the grinding traces, assembling the prisms into a single block of suitable size. The block may also be assembled from semiconductor rods prepared by precipitation out of the gaseous phase. Such rods generally have six cornered cross sections and can easily be assembled adjacent to each other.

The seams or joints at which the individual pieces engage each other need not be completely tight because the melt, due to capillary forces, will not fiow through the slits if they are not wider than 2 or 3 mm. Larger slits would soon be plugged with hardened semiconductor material from the melt. This hardened material is suflicient itself to hold the pieces together into a single block. Thus the wide slits are advantageous.

Because of the wide slits it is unnecessary to hold the block together by outside means after sufficient material has been melted by coils 4 and 5 to cause adherence into a single block. Before that time the loosely assembled block 2 may be held together in an insulating support structure. The support may be removed after adherence of the block pieces, or it may also be permitted to remain around the block 2. The block pieces may also be made to form a single block prior to the beginning of the crystal growing process by radiant heating.

After assembling block 2 from squared shapes, or from other constituents, the two heating coils 4, are located just above the block so as to produce on the upper surface below the induction heating coils a melt comprised of two sections.

A monocrystal seed is now dipped into the melt and withdrawn slowly in the known manner. Molten material solidifies on the seed as it is withdrawn. The solidified material forms a large single crystal. It also lowers the melt level so that the coils 4, 5 can be lowered to melt more block material. This is continued until the conditions existing in FIGS. 1 and 2 are approximated.

From this point on semiconductor material for formation of the semiconductor rod 6 is extracted only from one side of the melt whereas polycrystalline semiconductor material to be converted is introduced from the rod 7 at the other side of the melt. The block 2 and the heating coils 4, 5- can then remain at rest. Obviously, the recess in which the melt rests can be produced in another way for example by mechanical removal or by providing for it while assembling the individual constituents out of which the block 2 is constructed.

The arrangement in FIGS. 1 and 2 can produce monocrystalline semiconductor rods, particularly silicon rods of diameters greater than 35 mm. Drawing semiconductor rod 6 out through the induction heating coil 5 and introducing semiconductor rod 7 through induction heating coil 4 reliably provides equal heating. The withdrawn ingot if desired, can be rotated about its axis by means 10 at a speed of 10 to 150 r.p.rn., preferably 40 r.p.m., to achieve thereby a symmetrical growth. The induction coil heating power approximates 5 kilowatts.

The techniques for drawing crystal from a melt are generally described in Preparation of Single Crystals by W. D. Lawson and S. Nielson, published in 1958 by Butterworths Scientific Publications, London; and in Handbook of Semiconductor Electronics, edited by Lloyd P. Hunter, published by the McGraw-Hill Book Company, Inc., New York, Toronto and London, 1956.

As shown in FIG. 3, which otherwise corresponds to FIG. 1, at the beginning of the process before heating, the pieced-together block may also be assembled in a crucible C which loosely supports the pieces and which limits heat losses. The crucible preferably is comprised of graphite or quartz.

The crucible C, only partially shown, which surrounds the semiconductor block in FIG. 3 and which is of the direct current and/or induction type, may also heat the semiconductor material to some extent throughout the crystal growing operation. This permits keeping the heat introduced by the heating coils 4, 5 comparatively small. These coils then melt the semiconductor material whereas the external crucible heating merely helps maintain the semiconductor material at a high temperature, for silicon, over a thousand degrees, for example 1200 C. However, it is essential that the block 2 remain solid at its outside at all times. Thus care must be taken to avoid heating the semiconductor material at the crucible wall to its melting point. At such high temperature diffusion of impurities greatly increases, and in the case of a graphite crucible holding a silicon block a chemical reaction forming silicon carbide results. The melt 3 itself must at all times be surrounded by the block of solid semiconductor material. The method of FIG. 3 otherwise corresponds to that of FIGS. 1 and 2 as to performance and results achieved.

The above-mentioned semiconductor rods prepared by precipitation out of the gaseous phase, from which the block 2 is assembled, have hexagonal cross sections and are referred to as hexrods.

While embodiments of the invention have been described in detail it will be obvious to those skilled in the art that the invention can be practiced otherwise.

We claim:

1. Method of producing a monocrystalline semiconductor member which comprises forming a depression in the top of a block of semiconductor material, inductively heating the bottom of the depression from a heat source within the depression to a temperature at which a semiconductor melt is formed in the interior of the block, drawing a portion of the semiconductor material out of the melt so that it COOls and solidifies to a monocrystalline rod having a diameter greater than 25 mm., feeding a mass of polycrystalline semiconductor material from above through the depression to the melt at the bottom of the depression for replacing the portion of semiconductor material drawn therefrom, and simultaneously inductively heating the mass of polycrystalline semiconductor material to its melting temperature as it is being fed through the depression.

2. Method of producing a monocrystalline semiconductor member which comprises forming a depression in the top of a block of semiconductor material, heating the block from a heat source outside the depression to a temperature below its melting point, additionally inductively heating the bottom of the depression from a heat source within the depression to a temperature at which a semiconductor melt is formed in the interior of the block, drawing a portion of the semiconductor material out of the melt so that it cools and solidifies to a monocrystalline rod having a diameter greater than 25 mm., feeding a specific mass of polycrystalline semiconductor material from above through the depression to the melt at the bottom of the depression for replacing the portion of semiconductor material drawn therefrom, and simultaneously inductively heating the mass of polycrystalline semiconductor material to its melting temperature as it is being fed through the depression.

3. Method of producing a monocrystalline semiconductor member which comprises forming a cavity in the top of a block of semiconductor material, inductively heating the bottom of the cavity from a heat source Within the cavity to a temperature at which a semiconductor melt is formed in the interior of the block, drawing a mass of the semiconductor material out of the melt at a rate at which it cools and solidifies to a monocrystalline rod, feeding a rod of polycrystalline semiconductor material from above through the cavity to the melt at the bottom of the cavity at a rate for replacing the mass of semiconductor material drawn therefrom, and simultaneously inductively heating the rod of polycrystalline semiconductor material to its melting temperature as it is being fed through the cavity. 2

References Cited by the Examiner UNITED STATES PATENTS 2,793,103 5/1957 Emeis 23301 2,858,199 10/1958 Larson 148l.6 2,890,139 6/1959 Shockley 23273 2,914,397 11/1959 Sterling 1481.6 2,977,258 3/1961 Dunkle 148-1.6 2,979,386 4/1961 Shockley et al 1481.6 2,999,737 9/1961 Siebertz 23273 3,084,037 4/1963 Smith -10 3,160,497 12/1964 Loung 1481.6

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

N. F. MARKVA, Assistant Examiner. 

1. METHOD OF PRODUCING A MONOCRYSTALLINE SEMICONDUCTOR MEMBER WHICH COMPRISES FORMING A DEPRESSION IN THE TOP OF A BLOCK OF SEMICONDUCTOR MATERIAL, INDUCTIVELY HEATING THE BOTTOM OF THE DEPRESSION FROM A HEAT SOURCE WITHIN THE DEPRESSION TO A TEMPERATURE AT WHICH A SEMICONDUCTOR MELT IS FORMED IN THE INTERIOR OF THE BLOCK, DRAWING A PORTION OF THE SEMICONDUCTOR MATERIAL OUT OF THE MELT SO THAT IT COOLS AND SOLIDFIES TO A MONOCRYSTALLINE ROD HAVING A DIAMETER GREATER THAN 25 MM., FEEDING A MASS OF POLYCRYSTALLINE SEMICONDUCTOR MATERIAL FROM ABOVE THROUGH THE DEPRESSION TO THE MELT AT THE BOTTON OF THE DEPRESSION FOR REPLACING THE PORTION OF SEMICONDUCTOR MATERIAL DRAWN THEREFROM, AND SIMULTANEOUSLY INDUCTIVELY HEATING THE MASS OF POLYCRYSTALLINE SEMICONDUCTOR MATERIAL TO ITS MELTING TEMPERATURE AS IT IS BEING FED THROUGH THE DEPRESSION. 